3GPP LTE (3rd Generation Partnership Project Long Term Evolution) employs SC-FDMA (Single Carrier-Frequency Division Multiple Access) as its uplink access scheme. SC-FDMA features a low PAPR (Peak to Average Power Ratio) achieved by the single-carrier scheme of SC-FDMA, flexible allocation of data to sub-carrier frequencies, resilience to multipath interference in frequency-domain signal processing on the receiving side, and the like.
In SC-FDMA, on the transmitting side, SC-FDMA signals are generated, for example, by converting time-domain symbols into frequency components by a DFT (Discrete Fourier Transform), mapping the frequency components to respective different sub-carriers, then, converting the mapped frequency components back into time-domain waveform by an IDFT (Inverse Discrete Fourier Transform), and adding a CP (Cyclic Prefix) to the time-domain signal. Correspondingly, on the receiving side, the time-domain signal is recovered by converting a time-domain signal from the transmitting side into frequency components by a DFT, performing frequency equalization processing on the frequency components, and performing an IDFT on the signals after the frequency equalization processing. As described above, in SC-FDMA, the DFT on the transmitting side (hereinafter referred to as a transmitting DFT) corresponds to the IDFT on the receiving side (hereinafter referred to as a receiving IDFT), and the IDFT on the transmitting side (hereinafter referred to as a transmitting IDFT) corresponds to the DFT on the receiving side (hereinafter referred to as a receiving DFT).
As a new control scheme for the code rate of turbo coding, puncturing in the frequency domain (frequency puncturing, which may hereinafter be abbreviated as FP) has been drawing a lot of attention (see NPL 1 and NPL 2, for example). The frequency puncturing is a puncturing scheme which is essentially used in SC-FDMA systems, and in which puncturing is performed on frequency-domain signals after the transmitting DFT.
Operations of the frequency puncturing and puncturing in the time domain (time puncturing, which may hereinafter be abbreviated as TP), which is the conventional control scheme for the code rate of turbo coding will be described below (see FIG. 1).
In the time puncturing, puncturing is performed on encoded bits in the time-domain immediately after turbo coding (i.e., before the transmitting DFT) on a bit basis. For example, in FIG. 1, the last two bits are punctured (thinned) among ten encoded bits. In contrast, in the frequency puncturing, puncturing is performed on puncturing target data in which the encoded bits are convoluted into a plurality of symbols in the frequency domain, on a symbol basis. For example, in FIG. 1, the two symbols that are highest in frequency are punctured (thinned) among eight symbols mapped to eight sub-carriers. Thus, in the time puncturing, the whole of some encoded bits (or symbols) in the time domain are completely punctured. In contrast, in the frequency puncturing, some components of each encoded bit are punctured to a similar extent respectively. In other words, contrary to the time puncturing, in the frequency puncturing, the whole of some encoded bits are not completely punctured.
In consequence, assuming that transmission power is the same (i.e., the power corresponding to the punctured components is the same), the frequency puncturing allows for more parity bits per transmission than the time puncturing. The frequency puncturing may therefore improve an error correction coding gain as compared to the time puncturing by an increase in the number of transmitted parity bits. However, since the total punctured transmission power is the same in the time puncturing and the frequency puncturing if the transmission power is the same, the transmission power for each bit of a signal after the frequency puncturing is less than that of a signal after the time puncturing.
In the frequency puncturing, some frequency components of each encoded bit are punctured between the transmitting DFT and the receiving IDFT (i.e., after the transmitting DFT and before the receiving IDFT). This deteriorates unitarity (orthogonality) between the DFT matrix used in the transmitting DFT and the IDFT matrix used in the receiving IDFT, resulting in inter-symbol interference. In contrast, in the time puncturing, some encoded bits are punctured before the transmitting DFT. Thus, the unitarity is retained between the DFT matrix used in the transmitting DFT and the IDFT matrix used in the receiving IDFT.
As described above, in the frequency puncturing and the time puncturing, there is a trade-off between “improving the error correction coding gain by an increase of the number of parity bits” and “retaining the unitarity between the DFT matrix on the transmitting side and the IDFT matrix on the receiving side.”
For example, in LTE, the number of bits punctured by the time puncturing (the number of punctures) N is calculated by the following expression 1:[1]Number of Punctures N=(TBS÷Code Rate Ro)−(Modulation Level×Number of Allocated RBs NPRB×Number of Sub-Carriers per Allocated RB×Number of SC-FDMA Symbols in Subframe)  (Expression 1)
In expression 1, TBS stands for Transport Block Size and denotes the number of bits input to a turbo coder. The code rate (original code rate) Ro is a code rate before puncturing. For example, in 3GPP LTE-Advanced (hereinafter referred to as LTE-Advanced), which enables faster communication than LTE, Ro=⅓. The number of allocated RBs NPRB denotes the number of RBs that are allocated to data to be transmitted.
For example, in LTE-Advanced, the code rate Ro, the number of SC-FDMA symbols in a subframe, and the number of sub-carriers per allocated RB used in expression 1 are predetermined by a system. In contrast, the TBS, modulation level, and the number of allocated RBs NPRB used in expression 1 are determined by a base station for each subframe and indicated to a terminal via a downlink control channel.
In LTE-Advanced, two tables shown in FIG. 2A and FIG. 2B are used to determine the TBS. FIG. 2A is a table that shows a relationship between the MCS (Modulation and Coding Scheme) index and the TBS index, and FIG. 2B is a table that shows a relationship between the TBS index, the number of allocated RBs NPRB, and the TBS. For example, the tables shown in FIG. 2A and FIG. 2B are shared between the base station and the terminal.
At first, the base station determines the MCS index and the number of allocated RBs NPRB depending on a received SINR, an amount of data of the terminal that requests data transmission and the like. The base station indicates the determined MCS index and the number of allocated RBs NPRB to the terminal. The base station and the terminal then determine the modulation scheme and the TBS index from the MCS index in reference to the table shown in FIG. 2A. Next, the base station and the terminal determine the TBS from the TBS index and the number of allocated RBs NPRB in reference to the table shown in FIG. 2B.
For example, the case where the MCS index is 11 and the number of allocated RBs NPRB is 50 will be described. The modulation scheme: 16-QAM and the TBS index=10, which corresponds to the MCS index=11, are determined first in reference to FIG. 2A. The TBS=8760, which corresponds to the TBS index=10 and the number of allocated RBs NPRB=50, is determined next in reference to FIG. 2B.
In this way, since the tables shown in FIG. 2A and FIG. 2B are shared between the base station and the terminal, the terminal can determine the TBS for the terminal when the base station feeds back only the MCS index and the number of allocated RBs NNRB to the terminal. In consequence, the terminal can determine all the parameters used for calculating the number of punctures N by expression 1.
PTL 2 describes the fact that the combination of time puncturing and frequency puncturing can provide better error rate performances than the time puncturing alone in a static (AWGN (Additive White Gaussian Noise)) channel. In contrast, PTL 2 also describes the fact that the combination of time puncturing and frequency puncturing may provide worse error rate performances than the time puncturing alone in a multipath fading channel.
As a related art to improve efficiency in the use of frequency resources, clustered SC-FDMA has been proposed. In clustered SC-FDMA, SC-FDMA symbols are divided into a plurality of clusters and the plurality of clusters are mapped to frequency resources. Each cluster includes a plurality of RBs (Resource Blocks). In clustered SC-FDMA, a base station determines a cluster size (the number of symbols included in a cluster) and cluster-allocated positions and indicates the determined information to a terminal via a downlink control channel. For example, the base station sets resources having a high average received SINR (Signal to Interference and Noise Ratio) as the cluster-allocated positions in the frequency domain. In contrast, the terminal determines the number of symbols corresponding to the cluster size indicated via the downlink control channel as the number of symbols that are input to the DFT (transmitting DFT). The terminal then maps clusters including the symbols after the DFT to the respective allocated positions indicated via the downlink control channel. The mapped clusters are transmitted after the IDFT.